This invention relates in general to electrical insulation for high voltage apparatus and more specifically to gaseous insulation comprising perfluorocompounds. In the prior art several fluorinated or chlorinated hydrocarbons have been proposed for use as electrical insulation. See, for example, British Pat. Nos. 525,244 and 671,780. In U.S. Pat. No. 2,221,670 to F. S. Cooper, fluoro- and chlorofluoromethanes in admixture with nitrogen are shown to provide electrical insulation. While Cooper theorized that electrical breakdown was in some way related to the attachment of free electrons or loss of energy by inelastic collisions, these mechanisms apparently do not play a role in his selection of gaseous insulation. U.S. Pat. No. 2,989,577 to Berg describes the use of a mixture of SF.sub.6 and F.sub.3 CSF.sub.5. This mixture was shown to be more effective than SF.sub.6 under comparable conditions. U.S. Pat. No. 3,154,592 to Hauptschein et al describes the use of CF.sub.4, C.sub.2 F.sub.6 and C.sub.3 F.sub.8, all saturated perfluorocarbons, as dielectric media. U.S. Pat. No. 3,281,521 to Wilson proposes a mixture of N.sub.2, CCl .sub.2 F.sub.2 and SF.sub.6 to provide a composition which has a dielectric strength substantially as high as pure SF.sub.6 but having lower cost. It was also proposed that C.sub.2 F.sub.6 could be added to the mixture instead of SF.sub.6. The selection of components was based upon the fact that nitrogen reduced the dielectric strength and CCl.sub.2 F.sub.2 increased the dielectric strength. CCl.sub.2 F.sub.2 could not be used alone as a dielectric because of its dew point. There was no showing of any synergistic effects of the gases for preventing discharge, only that certain mixtures comprising a small amount of CCl.sub.2 F.sub.2 could provide dielectric strength substantially equivalent to SF.sub.6 at an economic advantage. U.S. Pat. No. 3,390,091 to Eibeck describes the use of a mixture of SF.sub.6 and thiazyltrifluoride. U.S. Pat. No. 3,506,774 to Gard et al proposed the use of perfluorovinylsulphurhexafluoride which could be added to SF.sub.6 or N.sub.2 . U.S. Pat. No. 3,650,955 to Manion describes the use of c-C.sub.4 F.sub.8 and C.sub.4 F.sub.10 an insulating materials.
Though many specific fluorinated compounds have been proposed for use as gaseous insulation material, there appears to be little knowledge in the art as to just what physicochemical properties of a gas or gaseous mixture are important in providing efficient insulation against high voltage discharge; consequently there is little guidance for the selection of alternate materials. In an article by M.S. Naidu et al published in J. Phys. D. (Appl. Phys.) 5, 741--746, 1972, the electrical properties of c-C.sub.4 F.sub.8 and iso-C.sub.4 F.sub.8 (perfluorobutene-2) are compared. It was postulated that the total electron attachment cross section for iso-C.sub.4 F.sub.8 was larger than that for c-C.sub.4 F.sub.8 thereby suggesting that the uniform field breakdown strength of c-C.sub.4 F.sub.8 would be substantially lower than that of iso-C.sub.4 F.sub.8. The double bond was thought to be responsible for the expected difference between the insulative properties of these structural isomers. There was no suggestion, however, of what the role of the double bonds might be in preventing breakdown. It can be seen that the selection of gaseous electrical insulators have heretofore been largely a matter of empirical experimentation; that is, several compounds are screaned and the ones showing the highest benefit/cost ratio are then proposed for use.